Gene Transfer of Connexin43 Mutants Attenuates Coupling in Cardiomyocytes

Fibroblasts Drive Metabolic Reprogramming in Pacemaker Cardiomyocytes

BACKGROUND

The sinoatrial node (SAN) is characterized by the microenvironment of pacemaker cardiomyocytes (PCs) encased with fibroblasts. An altered microenvironment leads to rhythm failure. Operable cell or tissue models are either generally lacking or difficult to handle. The biological process behind the milieu of SANs to evoke pacemaker rhythm is unknown. We explored how fibroblasts interact with PCs and regulate metabolic reprogramming and rhythmic activity in the SAN.

METHODS

Tbx18 (T-box transcription factor 18)-induced PCs and fibroblasts were used for cocultures and engineered tissues, which were used as the in vitro models to explore how fibroblasts regulate the functional integrity of SANs. RNA-sequencing, metabolomics, and cellular and molecular techniques were applied to characterize the molecular signals underlying metabolic reprogramming and identify its critical regulators. These pathways were further validated in vivo in rodents and induced human pluripotent stem cell-derived cardiomyocytes.

RESULTS

We observed that rhythmicity in Tbx18-induced PCs was regulated by aerobic glycolysis. Fibroblasts critically activated metabolic reprogramming and aerobic glycolysis within PCs, and, therefore, regulated pacemaker activity in PCs. The metabolic reprogramming was attributed to the exclusive induction of Aldoc (aldolase c) within PCs after fibroblast-PC integration. Fibroblasts activated the integrin-dependent mitogen-activated protein kinase-E2F1 signal through cell-cell contact and turned on Aldoc expression in PCs. Interruption of fibroblast-PC interaction or Aldoc knockdown nullified electrical activity. Engineered Tbx18-PC tissue sheets were generated to recapitulate the microenvironment within SANs. Aldoc-driven rhythmic machinery could be replicated within tissue sheets. Similar machinery was faithfully validated in de novo PCs of adult mice and rats, and in human PCs derived from induced pluripotent stem cells.

CONCLUSIONS

Fibroblasts drive Aldoc-mediated metabolic reprogramming and rhythmic regulation in SANs. This work details the cellular machinery behind the complex milieu of vertebrate SANs and opens a new direction for future therapy.

Biomaterial-induced conversion of quiescent cardiomyocytes into pacemaker cells in rats

Pacemaker cells can be differentiated from stem cells or transdifferentiated from quiescent mature cardiac cells via genetic manipulation. Here we show that the exposure of rat quiescent ventricular cardiomyocytes to a silk-fibroin hydrogel activates the direct conversion of the quiescent cardiomyocytes to pacemaker cardiomyocytes by inducing the ectopic expression of the vascular endothelial cell-adhesion glycoprotein cadherin. The silk-fibroin-induced pacemaker cells exhibited functional and morphological features of genuine sinoatrial-node cardiomyocytes in vitro, and pacemaker cells generated via the injection of silk fibroin in the left ventricles of rats functioned as a surrogate in situ sinoatrial node. Biomaterials with suitable surface structure, mechanics and biochemistry could facilitate the scalable production of biological pacemakers for human use.

Making better scar: Emerging approaches for modifying mechanical and electrical properties following infarction and ablation

Following myocardial infarction (MI), damaged myocytes are replaced by collagenous scar tissue, which serves an important mechanical function - maintaining integrity of the heart wall against enormous mechanical forces - but also disrupts electrical function as structural and electrical remodeling in the infarct and borderzone predispose to re-entry and ventricular tachycardia. Novel emerging regenerative approaches aim to replace this scar tissue with viable myocytes. Yet an alternative strategy of therapeutically modifying selected scar properties may also prove important, and in some cases may offer similar benefits with lower risk or regulatory complexity. Here, we review potential goals for such modifications as well as recent proof-of-concept studies employing specific modifications, including gene therapy to locally increase conduction velocity or prolong the refractory period in and around the infarct scar, and modification of scar anisotropy to improve regional mechanics and pump function. Another advantage of scar modification techniques is that they have applications well beyond MI. In particular, ablation treats electrical abnormalities of the heart by intentionally generating scar to block aberrant conduction pathways. Yet in diseases such as atrial fibrillation (AF) where ablation can be extensive, treating the electrical disorder can significantly impair mechanical function. Creating smaller, denser scars that more effectively block conduction, and choosing the location of those lesions by balancing their electrical and mechanical impacts, could significantly improve outcomes for AF patients. We review some recent advances in this area, including the use of computational models to predict the mechanical effects of specific lesion sets and gene therapy for functional ablation. Overall, emerging techniques for modifying scar properties represents a potentially important set of tools for improving patient outcomes across a range of heart diseases, whether used in place of or as an adjunct to regenerative approaches.

Cx43 phosphorylation on S279/282 and intercellular communication are regulated by IP3/IP3 receptor signaling

Background

Inositol 1,4,5-trisphosphate receptor (IP₃R) plays a pivotal role in the Ca²⁺ release process in a variety of cell types. Additionally, IP₃R is distributed in ventricular intercalated discs, but its function(s) in this particular site remains unknown. Connexin (Cx43), the predominant gap junction (GJ) protein in ventricular myocardium, is linked to several signaling pathways that regulate Cx43 properties by (de)phosphorylation on multiple residues. Here, we investigated the regulatory role of IP₃R in cell-cell communication and the mechanism(s) underlying this effect.

Results

In neonatal rat and adult mouse ventricular myocytes IP₃R co-localized and co-immunoprecipitated with Cx43 in GJ plaques detected by immunostaining and western blot assays. Blocking IP₃R with antagonists or silencing pan-IP₃R expression with shRNA hindered the 6-carboxyfluorescein (6-CFDA) diffusion through GJs and desynchronized Ca²⁺ transients among confluent neonatal myocytes in culture, whereas stimulation of IP₃R with IP₃ ester or ATP exerted the opposite effect. Likewise, 6-CFDA propagation through GJs was modulated by IP₃R activation or inhibition in cell pairs of isolated adult cardiomyocytes. Furthermore, IP₃R activation or IP₃R suppression promoted or suppressed, respectively, Cx43 phosphorylation on S279/282. Site-directed mutagenesis indicated that expression of a mutant Cx43-S282A (alanine) inhibited S279/282 phosphorylation and GJ permeability, while the S279A mutant showed the opposite effect in ventricular myocytes. Expression of these mutants in HEK293 cells revealed that cells with a dual S279/282 mutation failed to express exogenous Cx43, whereas cells with a single S279 or S282 mutation displayed Cx43 overexpression with increased phosphorylation of S279/282 and promotion of intercellular communication.

Conclusions

These results demonstrated, for the first time, that IP₃R physically interacts with Cx43 and participates in the regulation of Cx43 phosphorylation on S279/282, thereby affecting GJ intercellular communication in ventricular myocytes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-014-0058-6) contains supplementary material, which is available to authorized users.

A mutation causing Brugada syndrome identifies a mechanism for altered autonomic and oxidant regulation of cardiac sodium currents

BACKGROUND

The mechanisms of the electrocardiographic changes and arrhythmias in Brugada syndrome (BrS) remain controversial. Mutations in the sodium channel gene, SCN5A, and regulatory proteins that reduce or eliminate sodium current (INa) have been linked to BrS. We studied the properties of a BrS-associated SCN5A mutation in a protein kinase A (PKA) consensus phosphorylation site, R526H.

METHODS AND RESULTS

In vitro PKA phosphorylation was detected in the I-II linker peptide of wild-type (WT) channels but not R526H or S528A (phosphorylation site) mutants. Cell surface expression of R526H and S528A channels was reduced compared with WT. Whole-cell INa through all channel variants revealed no significant differences in the steady-state activation, inactivation, and recovery from inactivation. Peak current densities of the mutants were significantly reduced compared with WT. Infection of 2D cultures of neonatal rat ventricular myocytes with WT and mutant channels increased conduction velocity compared with noninfected cells. PKA stimulation significantly increased peak INa and conduction velocity of WT but not mutant channels. Oxidant stress inhibits cardiac INa; WT and mutant INa decreases with the intracellular application of reduced nicotinamide adenine dinucleotide (NADH), an effect that is reversed by PKA stimulation in WT but not in R526H or S528A channels.

CONCLUSIONS

We identified a family with BrS and an SCN5A mutation in a PKA consensus phosphorylation site. The BrS mutation R526H is associated with a reduction in the basal level of INa and a failure of PKA stimulation to augment the current that may contribute to the predisposition to arrhythmias in patients with BrS, independent of the precipitants.

From stem cells to cardiomyocytes: the role of forces in cardiac maturation, aging, and disease

Stem cell differentiation into a variety of lineages is known to involve signaling from the extracellular niche, including from the physical properties of that environment. What regulates stem cell responses to these cues is there ability to activate different mechanotransductive pathways. Here, we will review the structures and pathways that regulate stem cell commitment to a cardiomyocyte lineage, specifically examining proteins within muscle sarcomeres, costameres, and intercalated discs. Proteins within these structures stretch, inducing a change in their phosphorylated state or in their localization to initiate different signals. We will also put these changes in the context of stem cell differentiation into cardiomyocytes, their subsequent formation of the chambered heart, and explore negative signaling that occurs during disease.

Origin, development, and differentiation of cardiac fibroblasts

Cardiac fibroblasts are the most abundant cell in the mammalian heart. While they have been historically underappreciated in terms of their functional contributions to cardiac development and physiology, they and their activated form, myofibroblasts, are now known to play key roles in both development and disease through structural, paracrine, and electrical interactions with cardiomyocytes. The lack of specific markers for fibroblasts currently convolutes the study of this dynamic cell lineage, but advances in marker analysis and lineage mapping technologies are continuously being made. Understanding how to best utilize these tools, both individually and in combination, will help to elucidate the functional significance of fibroblast-cardiomyocyte interactions in vivo. Here we review what is currently known about the diverse roles played by cardiac fibroblasts and myofibroblasts throughout development and periods of injury with the intent of emphasizing the duality of their nature. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium ".

Engraftment patterns of human adult mesenchymal stem cells expose electrotonic and paracrine proarrhythmic mechanisms in myocardial cell cultures

BACKGROUND

After intramyocardial injection, mesenchymal stem cells (MSCs) may engraft and influence host myocardium. However, engraftment rate and pattern of distribution are difficult to control in vivo, hampering assessment of potential adverse effects. In this study, the role of the engraftment patterns of MSCs on arrhythmicity in controllable in vitro models is investigated.

METHODS AND RESULTS

Cocultures of 4×10(5) neonatal rat cardiomyocytes and 7% or 28% adult human MSCs (hMSCs) in diffuse or clustered distribution patterns were prepared. Electrophysiological effects were studied by optical mapping and patch-clamping. In diffuse cocultures, hMSCs dose-dependently decreased neonatal rat cardiomyocyte excitability, slowed conduction, and prolonged action potential duration until 90% repolarization (APD90). Triggered activity (14% versus 0% in controls) and increased inducibility of re-entry (53% versus 6% in controls) were observed in 28% hMSC cocultures. MSC clusters increased APD90, slowed conduction locally, and increased re-entry inducibility (23%), without increasing triggered activity. Pharmacological heterocellular electric uncoupling increased excitability and conduction velocity to 133% in 28% hMSC cocultures, but did not alter APD90. Transwell experiments showed that hMSCs dose-dependently increased APD90, APD dispersion, inducibility of re-entry and affected specific ion channel protein levels, whereas excitability was unaltered. Incubation with hMSC-derived exosomes did not increase APD in neonatal rat cardiomyocyte cultures.

CONCLUSIONS

Adult hMSCs affect arrhythmicity of neonatal rat cardiomyocyte cultures by heterocellular coupling leading to depolarization-induced conduction slowing and by direct release of paracrine factors that negatively affect repolarization rate. The extent of these detrimental effects depends on the number and distribution pattern of hMSCs. These results suggest that caution should be urged against potential adverse effects of myocardial hMSC engraftment.

Targeted MicroRNA Interference Promotes Postnatal Cardiac Cell Cycle Re-Entry

Mammalian heart cells undergo a marked reduction in proliferative activity shortly after birth, and thereafter grow predominantly by hypertrophy. Our understanding of the molecular mechanisms underlying cardiac maturation and senescence is based largely on studies at the whole-heart level. Here, we investigate the molecular basis of the acquired quiescence of purified neonatal and adult cardiomyocytes, and use microRNA interference as a novel strategy to promote cardiomyocyte cell cycle re-entry. Expression of cyclins and cyclin-dependent kinases (CDKs) and positive modulators were down-regulated, while CDK inhibitors and negative cell cycle modulators were up-regulated during postnatal maturation of cardiomyocytes. The expression pattern of microRNAs also changed dramatically, including increases in miR-29a, miR-30a and miR-141. Treatment of neonatal cardiomyocytes with miRNA inhibitors anti-miR-29a, anti-miR-30a, and antimiR-141 resulted in more cycling cells and enhanced expression of Cyclin A2 (CCNA2). Thus, targeted microRNA interference can reactivate postnatal cardiomyocyte proliferation.

Cell and gene therapy for arrhythmias: Repair of cardiac conduction damage

Action potentials generated in the sinoatrial node (SAN) dominate the rhythm and rate of a healthy human heart. Subsequently, these action potentials propagate to the whole heart via its conduction system. Abnormalities of impulse generation and/or propagation in a heart can cause arrhythmias. For example, SAN dysfunction or conduction block of the atrioventricular node can lead to serious bradycardia which is currently treated with an implanted electronic pacemaker. On the other hand, conduction damage may cause reentrant tachyarrhythmias which are primarily treated pharmacologically or by medical device-based therapies, including defibrillation and tissue ablation. However, drug therapies sometimes may not be effective or are associated with serious side effects. Device-based therapies for cardiac arrhythmias, even with well developed technology, still face inadequacies, limitations, hardware complications, and other challenges. Therefore, scientists are actively seeking other alternatives for antiarrhythmic therapy. In particular, cells and genes used for repairing cardiac conduction damage/defect have been investigated in various studies both in vitro and in vivo. Despite the complexities of the excitation and conduction systems of the heart, cell and gene-based strategies provide novel alternatives for treatment or cure of cardiac arrhythmias. This review summarizes some highlights of recent research progress in this field.

Pluripotent stem cell-derived cardiac tissue patch with advanced structure and function

Recent advances in pluripotent stem cell research have provided investigators with potent sources of cardiogenic cells. However, tissue engineering methodologies to assemble cardiac progenitors into aligned, 3-dimensional (3D) myocardial tissues capable of physiologically relevant electrical conduction and force generation are lacking. In this study, we introduced 3D cell alignment cues in a fibrin-based hydrogel matrix to engineer highly functional cardiac tissues from genetically purified mouse embryonic stem cell-derived cardiomyocytes (CMs) and cardiovascular progenitors (CVPs). Procedures for CM and CVP derivation, purification, and functional differentiation in monolayer cultures were first optimized to yield robust intercellular coupling and maximize velocity of action potential propagation. A versatile soft-lithography technique was then applied to reproducibly fabricate engineered cardiac tissues with controllable size and 3D architecture. While purified CMs assembled into a functional 3D syncytium only when supplemented with supporting non-myocytes, purified CVPs differentiated into cardiomyocytes, smooth muscle, and endothelial cells, and autonomously supported the formation of functional cardiac tissues. After a total culture time similar to period of mouse embryonic development (21 days), the engineered cardiac tissues exhibited unprecedented levels of 3D organization and functional differentiation characteristic of native neonatal myocardium, including: 1) dense, uniformly aligned, highly differentiated and electromechanically coupled cardiomyocytes, 2) rapid action potential conduction with velocities between 22 and 25 cm/s, and 3) significant contractile forces of up to 2 mN. These results represent an important advancement in stem cell-based cardiac tissue engineering and provide the foundation for exploiting the exciting progress in pluripotent stem cell research in the future tissue engineering therapies for heart disease.

Human dermal fibroblasts derived from oculodentodigital dysplasia patients suggest that patients may have wound-healing defects

Oculodentodigital dysplasia (ODDD) is primarily an autosomal dominant human disease caused by any one of over 60 mutations in the GJA1 gene encoding the gap junction protein Cx43. In the present study, wound healing was investigated in a G60S ODDD mutant mouse model and by using dermal fibroblasts isolated from two ODDD patients harboring the p.D3N and p.V216L mutants along with dermal fibroblasts isolated from their respective unaffected relatives. Punch biopsies revealed a delay in wound closure in the G60S mutant mice in comparison to wild-type littermates, and this delay appeared to be due to defects in the dermal fibroblasts. Although both the p.D3N and p.V216L mutants reduced gap junctional intercellular communication in human dermal fibroblasts, immunolocalization studies revealed that Cx43 gap junctions were prevalent at the cell surface of p.D3N expressing fibroblasts but greatly reduced in p.V216L expressing fibroblasts. Mutant expressing fibroblasts were further found to have reduced proliferation and migration capabilities. Finally, in response to TGFβ1, mutant expressing fibroblasts expressed significantly less alpha smooth muscle actin suggesting they were inefficient in their ability to differentiate into myofibroblasts. Collectively, our results suggest that ODDD patients may have subclinical defects in wound healing due to impaired function of dermal fibroblasts.

Connexin43 hemichannels contribute to cadmium-induced oxidative stress and cell injury

We investigated the potential involvement of connexin hemichannels in cadmium ions (Cd(2+))-elicited cell injury. Transfection of LLC-PK1 cells with a wild-type connexin43 (Cx43) sensitized them to Cd(2+)-elicited cell injury. The cell susceptibility to Cd(2+) was increased by depletion of glutathione (GSH) with DL-buthionine-[S,R]-sulfoximine, and decreased by N-acetyl-cysteine or glutathione reduced ethyl ester. Fibroblasts derived from Cx43 wild-type (Cx43+/+) and knockout (Cx43-/-) fetal littermates displayed different susceptibility to Cd(2+). Cd(2+) induced a higher concentration of reactive oxygen species, a stronger activation c-Jun N-terminal kinase, and significantly more severe cell injury in Cx43+/+ fibroblasts, as compared with Cx43-/- fibroblasts. Cd(2+) caused a reduction in intracellular GSH, whereas it elevated extracellular GSH. This effect of Cd(2+) was more dramatic in Cx43+/+ than Cx43-/- fibroblasts. Treatment of Cx43+/+ fibroblasts with Cd(2+) caused a Cx43 hemichannel-dependent influx of Lucifer Yellow and efflux of ATP. Collectively, our study demonstrates that Cx43 sensitizes cells to Cd(2+)-initiated cytotoxicity, possibly through hemichannel-mediated effects on intracellular oxidative status.

Constitutive HIF-1α expression blunts the beneficial effects of cardiosphere-derived cell therapy in the heart by altering paracrine factor balance

Hypoxia-inducible factor-1alpha (HIF-1α) expression promotes angiogenesis and can influence stem cell engraftment. We investigated the effect of stable over-expression of constitutively active HIF-1α on cardiosphere-derived cell (CDC) engraftment and left ventricular function. CDCs were transduced with a lentivirus expressing a constitutively active mutant of human HIF-1α (LVHIF-1α). Two million male rat CDCs were injected into the infarct following ligation of the mid-LAD in female syngeneic rats. Left ventricular ejection fraction (EF) and circumferential strain were measured by echocardiography at 1 and 4 weeks post-MI in the following groups: PBS group (n = 7), CELL group (n = 7), and CELL-HIF group (n = 7). HIF-1α, VEGF, endothelin-1 expression, and CDC engraftment were measured by quantitative PCR. At 30 days, EF was unchanged in the CELL-HIF group (p = NS), increased in the CELL group (p = 0.025), and decreased in the PBS group (p = 0.021), but engraftment was similar (2.4% ± 3.3% vs 1.7% ± 0.8%, p = NS). Mean circumferential strain of the infarcted region was unchanged in the CELL-HIF group, but improved in the CELL group (p = 0.02). Endothelin-1 and VEGF expression were higher in HIF-CDCs exposed to hypoxia, compared with non-transduced CDCs. HIF-1α expression in CDCs blunted the beneficial functional effects of CDC transplantation, suggesting that paracrine factor balance may play an important role in cardiac regeneration.

Isolation and expansion of functionally-competent cardiac progenitor cells directly from heart biopsies

The adult heart contains reservoirs of progenitor cells that express embryonic and stem cell-related antigens. While these antigenically-purified cells are promising candidates for autologous cell therapy, clinical application is hampered by their limited abundance and tedious isolation methods. Methods that involve an intermediate cardiosphere-forming step have proven successful and are being tested clinically, but it is unclear whether the cardiosphere step is necessary. Accordingly, we investigated the molecular profile and functional benefit of cells that spontaneously emigrate from cardiac tissue in primary culture. Adult Wistar-Kyoto rat hearts were minced, digested and cultured as separate anatomical regions. Loosely-adherent cells that surround the plated tissue were harvested weekly for a total of five harvests. Genetic lineage tracing demonstrated that a small proportion of the direct outgrowth from cardiac samples originates from myocardial cells. This outgrowth contains sub-populations of cells expressing embryonic (SSEA-1) and stem cell-related antigens (c-Kit, abcg2) that varied with time in culture but not with the cardiac chamber of origin. This direct outgrowth, and its expanded progeny, underwent marked in vitro angiogenic/cardiogenic differentiation and cytokine secretion (IGF-1, VGEF). In vivo effects included long-term functional benefits as gauged by MRI following cell injection in a rat model of myocardial infarction. Outgrowth cells afforded equivalent functional benefits to cardiosphere-derived cells, which require more processing steps to manufacture. These results provide the basis for a simplified and efficient process to generate autologous cardiac progenitor cells (and mesenchymal supporting cells) to augment clinically-relevant approaches for myocardial repair.

Biological therapies for cardiac arrhythmias: can genes and cells replace drugs and devices?

Cardiac rhythm disorders reflect failures of impulse generation and/or conduction. With the exception of ablation methods that yield selective endocardial destruction, present therapies are nonspecific and/or palliative. Progress in understanding the underlying biology opens up prospects for new alternatives. This article reviews the present state of the art in gene- and cell-based therapies to correct cardiac rhythm disturbances. We begin with the rationale for such approaches, briefly discuss efforts to address aspects of tachyarrhythmia, and review advances in creating a biological pacemaker to cure bradyarrhythmia. Insights gained bring the field closer to a paradigm shift away from devices and drugs, and toward biologics, in the treatment of rhythm disorders.

TVP1022 protects neonatal rat ventricular myocytes against doxorubicin-induced functional derangements

Our recent studies demonstrated that propargylamine derivatives such as rasagiline (Azilect, Food and Drug Administration-approved anti-Parkinson drug) and its S-isomer TVP1022 protect cardiac and neuronal cell cultures against apoptotic-inducing stimuli. Studies on structure-activity relationship revealed that their neuroprotective effect is associated with the propargylamine moiety, which protects mitochondrial viability and prevents apoptosis by activating Bcl-2 and protein kinase C-epsilon and by down-regulating the proapoptotic protein Bax. Based on the established cytoprotective and neuroprotective efficacies of propargylamine derivatives, as well as on our recent study showing that TVP1022 attenuates serum starvation-induced and doxorubicin-induced apoptosis in neonatal rat ventricular myocytes (NRVMs), we tested the hypothesis that TVP1022 will also provide protection against doxorubicin-induced NRVM functional derangements. The present study demonstrates that pretreatment of NRVMs with TVP1022 (1 microM, 24 h) prevented doxorubicin (0.5 microM, 24 h)-induced elevation of diastolic [Ca(2+)](i), the slowing of [Ca(2+)](i) relaxation kinetics, and the decrease in the rates of myocyte contraction and relaxation. Furthermore, pretreatment with TVP1022 attenuated the doxorubicin-induced reduction in the protein expression of sarco/endoplasmic reticulum calcium (Ca(2+)) ATPase, Na(+)/Ca(2+) exchanger 1, and total connexin 43. Finally, TVP1022 diminished the inhibitory effect of doxorubicin on gap junctional intercellular coupling (measured by means of Lucifer yellow transfer) and on conduction velocity, the amplitude of the activation phase, and the maximal rate of activation (dv/dt(max)) measured by the Micro-Electrode-Array system. In summary, our results indicate that TVP1022 acts as a novel cardioprotective agent against anthracycline cardiotoxicity, and therefore potentially can be coadmhence, theinistered with doxorubicin in the treatment of malignancies in humans.

Connexin43 knockdown or overexpression modulates cell coupling in control and failing rabbit left ventricular myocytes

AIMS

We have shown that failing human and rabbit left ventricle (LV) exhibits downregulation and dephosphorylation of connexin43 (Cx43) and that Cx43 dephosphorylation in heart failure (HF) contributes to reduced cell coupling. However, the role of Cx43 downregulation per se in impaired coupling in HF is unclear.

METHODS AND RESULTS

First, we used adenovirus (Ad) encoding a Cx43 siRNA sequence to knock down Cx43 protein levels in cultured control rabbit LV myocytes. Cells cultured for up to 48 h with intermittent pacing maintained Cx43 protein levels and phosphorylation status. Cell coupling in Cx43 knockdown myocyte pairs (by Lucifer Yellow dye transfer) was markedly reduced after 24 h infection (associated with approximately 40% Cx43 knockdown) and after 48 h (associated with approximately 70% Cx43 knockdown). The phosphorylation status, distribution of remaining Cx43 proteins, and levels of other cardiac connexins (Cx40 and Cx45) were unchanged. Second, we overexpressed Cx43 to levels comparable to control using an adenovirus encoding wild-type Cx43 (Cx43WT) gene in isolated LV myocytes from our arrhythmogenic HF rabbit model. We found 87% more Cx43WT proteins improved dye coupling [vs. Ad-beta-galactosidase (LacZ) infected HF controls]. Overexpressed Cx43 protein was located throughout the myocyte membrane (same pattern as in controls), and the phosphorylation status of Cx43 remained comparable to that in AdLacZ infected HF controls.

CONCLUSION

In addition to Cx43 dephosphorylation, downregulation of Cx43 plays an essential role in reduced cell coupling in the failing rabbit heart. Modulation of Cx43 expression could be a novel therapeutic approach to improve conduction and decrease sudden death in HF.

Modification of cardiovascular ion channels by gene therapy

Delivery of genes to the heart and vasculature for therapeutic purposes is an exciting strategy that is approaching clinical reality. Abnormalities of expression or function of ion channels is central to many cardiovascular diseases and gene delivery to modify ion channels is an appealing alternative to traditional therapy with small-molecule drugs. Potential therapeutic targets include hypertrophy and heart failure, atrioventricular node modification in atrial fibrillation, ventricular tachycardia and hypertension. Numerous approaches for gene delivery are under development, including use of tissue-specific promoters in viral vectors. For other applications, such as development of biological pacemakers, cells can be transduced with pacemaker genes in vitro, and then the cells implanted within the heart. There are short-term hurdles to therapeutic gene delivery to modify cardiovascular ion channels, but in the intermediate and longer term, the outlook is promising.

Non-cell-autonomous effects of vector-expressed regulatory RNAs in mammalian heart cells

In mammalian cells, small regulatory RNA molecules are able to modulate gene expression in a cell-autonomous manner. In contrast, this mechanism of gene regulation can occur systemically in plants and nematodes. The existence of similar cell-to-cell transmission in mammalian cells has been explored, but generalizibilty and mechanistic insights have remained elusive. Here, we show that small regulatory RNA molecules are capable of a non-cell-autonomous effect between primary cardiac myocytes through a gap-junction-dependent mechanism. Co-culture experiments showed that both Dicer-processed small-interfering RNAs (siRNAs) and Drosha-processed microRNAs (miRNAs) were capable of target gene knockdown and physiological effects in a non-cell-autonomous manner. Target gene siRNA molecules were detected in recipient cells, indicating transfer of the primary effector molecule. All of these effects were abrogated by dominant-negative molecular suppression of gap junction function. Our results show that both siRNAs and miRNAs are capable of a non-cell-autonomous effect between mammalian cells through gap junctions. The recognition of this biological process raises the novel therapeutic prospect of a bystander effect after gene transfer to tissues bearing gap junctions and for cell engineering with a view to creating regulatory RNA donor cells that exert their influence throughout a syncytium.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

Biological therapies for atrial fibrillation

Atrial fibrillation is a prominent cause of morbidity and mortality in developed countries. Current treatment strategies center on controlling heart rate while allowing fibrillation to persist or targeting fibrillation primarily and attempting to maintain sinus rhythm. Pharmacological therapies are largely successful for rate control, although mild toxicities are common. Rhythm control strategies are often unsuccessful, leaving patients in atrial fibrillation despite attempts to maintain sinus rhythm. This review will discuss novel biological strategies that are currently under development and may eventually have impact on the management of atrial fibrillation.

Calmodulin kinase II inhibition disrupts cardiomyopathic effects of enhanced green fluorescent protein

Transgenic expression of enhanced green fluorescent protein (eGFP) in myocardium can result in cardiac dysfunction and cardiomyopathy, presumably through toxic effects that disrupt normal cellular signaling. The multifunctional Ca(2+)- and calmodulin-dependent protein kinase II (CaMKII) is widely expressed in myocardium and CaMKII activity is increased in human and animal models of cardiomyopathy, so we hypothesized that increased CaMKII activity is important for cardiomyopathy due to transgenic expression of eGFP. Here we report that cardiomyocyte-delimited eGFP over-expression causes increased CaMKII activity that predicts left ventricular dilation and dysfunction. On the other hand, transgenic co-expression of a CaMKII inhibitory peptide with eGFP prevents eGFP-mediated left ventricular dilation and dysfunction. These findings suggest that increased CaMKII activity is a critical pathological signal in transgenic cardiomyopathy due to eGFP over-expression.

Lentiviral vector-mediated expression of GFP or Kir2.1 alters the electrophysiology of neonatal rat ventricular myocytes without inducing cytotoxicity

Recombinant lentiviral vectors (LVs) are capable of transducing neonatal rat ventricular myocytes (NRVMs) and providing stable, long-term transgene expression. The goal of the present study was to comprehensively test whether transduction of NRVMs by LVs results in cytotoxicity and to examine the electrophysiological consequences of gene modification of NRVM monolayers by two vectors: one encoding a putatively inert enhanced green fluorescent protein (eGFP) and the other a major ion channel protein, inward rectifier K+channel (Kir) 2.1. Freshly isolated NRVMs were transduced and cultured in monolayers. Immunohistochemistry, Trypan blue exclusion, annexin V binding followed by flow cytometry (FCM), and terminal transferase dUTP nick-end labeling assays were performed to assess for cytotoxicity. Optical mapping studies of action potential propagation in NRVM monolayers were performed to characterize the electrophysiological alterations following transduction. The cytotoxicity assays revealed that transduction had no adverse effects on NRVM cultures. However, eGFP-transduced monolayers exhibited a decrease in conduction velocity (CV) and action potential duration (APD) compared with monolayers transduced with LVs encoding LacZ or devoid of a transgene. In addition, small interfering RNA-mediated knockdown of eGFP expression corrected this phenotype. In contrast, Kir2.1 gene-modified monolayers showed an increase in CV and a predictable decrease in APD. This study demonstrates that LVs transduce NRVMs without cytotoxic effects. However, eGFP has a significant effect on APD and CV in this experimental system and calls into question the widely held belief that GFP is physiologically inert. In addition, LV-mediated overexpression of Kir2.1 opens up the prospect of studying the functional role of inward rectifier K+current in cardiac arrhythmias.